Introduction 8.1

Unit protection schemes 8.2

Teleprotection commands 8.3

Intertripping 8.4

Performance requirements 8.5

Transmission media, interference and noise 8.6

Methods of signalling 8.7

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8.1 INTRODUCTION

Unit protection schemes, formed by a number of relayslocated remotely from each other, and some distanceprotection schemes, require some form of communicationbetween each location in order to achieve a unit protectionfunction. This form of communication is known asprotection signalling. Additionally communicationsfacilities are also required when remote operation of acircuit breaker is required as a result of a local event. Thisform of communications is known as intertripping.

The communication messages involved may be quitesimple, involving instructions for the receiving device totake some defined action (trip, block, etc.), or it may bethe passing of measured data in some form from onedevice to another (as in a unit protection scheme).

Various types of communication links are available forprotection signalling, for example:

i. private pilot wires installed by the powerauthority

ii. pilot wires or channels rented from acommunications company

iii. carrier channels at high frequencies over thepower lines

iv. radio channels at very high or ultra highfrequencies

v. optical fibres

Whether or not a particular link is used depends onfactors such as the availability of an appropriatecommunication network, the distance betweenprotection relaying points, the terrain over which thepower network is constructed, as well as cost.

Protection signalling is used to implement UnitProtection schemes, provide teleprotection commands,or implement intertripping between circuit breakers.

8.2 UNIT PROTECTION SCHEMES

Phase comparison and current differential schemes usesignalling to convey information concerning the relayingquantity - phase angle of current and phase and

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magnitude of current respectively - between local andremote relaying points. Comparison of local and remotesignals provides the basis for both fault detection anddiscrimination of the schemes.

Details of Unit Protection schemes are given in Chapter 10.Communications methods are covered later in this Chapter.

8.3 TELEPROTECTION COMMANDS

Some Distance Protection schemes described in Chapter12 use signalling to convey a command between localand remote relaying points. Receipt of the informationis used to aid or speed up clearance of faults within aprotected zone or to prevent tripping from faults outsidea protected zone.

Teleprotection systems are often referred to by theirmode of operation, or the role of the teleprotectioncommand in the system.

8.4 INTERTRIPPING

Intertripping is the controlled tripping of a circuitbreaker so as to complete the isolation of a circuit or

piece of apparatus in sympathy with the tripping of othercircuit breakers. The main use of such schemes is toensure that protection at both ends of a faulted circuitwill operate to isolate the equipment concerned. Possiblecircumstances when it may be used are:

a. a feeder with a weak infeed at one end, insufficientto operate the protection for all faults

b. feeder protection applied to transformer –feedercircuits. Faults on the transformer windings mayoperate the transformer protection but not thefeeder protection. Similarly, some earth faults maynot be detected due to transformer connections

c. faults between the CB and feeder protection CT’s,when these are located on the feeder side of the CB.Bus-zone protection does not result in faultclearance – the fault is still fed from the remote endof the feeder, while feeder unit protection may notoperate as the fault is outside the protected zone

d. some distance protection schemes useintertripping to improve fault clearance times forsome kinds of fault – see Chapters 12/13

Intertripping schemes use signalling to convey a trip

Power transmission line

Figure 8.1: Application of protection signalling and its relationship to other systems using communication(Shown as a unidirectional system for simplicity)

Blocking Communicationlink

VI

Data

Communicationsystems

Telecontrol

Telephone

Permissivetrip

Telemetry

Data

Communicationsystems

Telecontrol

Telephone

Telemetry

Teleprotectioncommand(receive)

Teleprotectioncommand

(send)

Protectionrelay

scheme

Protectionrelay

scheme

V ITripTrip

Intertrip

Blocking

Permissivetrip

Intertrip

Figure 8.1: Application of protection signalling and its relationship to other systems using communication(shown as a unidirectional system for simplicity)

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command to remote circuit breakers to isolate circuits.For high reliability EHV protection schemes, intertrippingmay be used to give back-up to main protections, orback-tripping in the case of breaker failure. Three typesof intertripping are commonly encountered, and aredescribed below.

8.4.1 Direct Tripping

In direct tripping applications, intertrip signals are sentdirectly to the master trip relay. Receipt of the commandcauses circuit breaker operation. The method ofcommunication must be reliable and secure because anysignal detected at the receiving end will cause a trip ofthe circuit at that end. The communications systemdesign must be such that interference on thecommunication circuit does not cause spurious trips.Should a spurious trip occur, considerable unnecessaryisolation of the primary system might result, which is atbest undesirable and at worst quite unacceptable.

8.4.2 Permissive Tripping

Permissive trip commands are always monitored by aprotection relay. The circuit breaker is tripped whenreceipt of the command coincides with operation of theprotection relay at the receiving end responding to asystem fault. Requirements for the communicationschannel are less onerous than for direct trippingschemes, since receipt of an incorrect signal mustcoincide with operation of the receiving end protectionfor a trip operation to take place. The intention of theseschemes is to speed up tripping for faults occurringwithin the protected zone.

8.4.3 Blocking Scheme

Blocking commands are initiated by a protection elementthat detects faults external to the protected zone.Detection of an external fault at the local end of aprotected circuit results in a blocking signal beingtransmitted to the remote end. At the remote end,receipt of the blocking signal prevents the remote endprotection operating if it had detected the external fault.Loss of the communications channel is less serious forthis scheme than in others as loss of the channel doesnot result in a failure to trip when required. However,the risk of a spurious trip is higher.

Figure 8.1 shows the typical applications of protectionsignalling and their relationship to other signallingsystems commonly required for control and managementof a power system. Of course, not all of the protectionsignals shown will be required in any particular scheme.

8.5 PERFORMANCE REQUIREMENTS

Overall fault clearance time is the sum of:

a. signalling time

b. protection relay operating time

c. trip relay operating time

d. circuit breaker operating time

The overall time must be less than the maximum time forwhich a fault can remain on the system for minimumplant damage, loss of stability, etc. Fast operation istherefore a pre-requisite of most signalling systems.

Typically the time allowed for the transfer of a commandis of the same order as the operating time of theassociated protection relays. Nominal operating timesrange from 5 to 40ms dependent on the mode ofoperation of the teleprotection system.

Protection signals are subjected to the noise andinterference associated with each communicationmedium. If noise reproduces the signal used to conveythe command, unwanted commands may be produced,whilst if noise occurs when a command signal is beingtransmitted, the command may be retarded or missedcompletely. Performance is expressed in terms ofsecurity and dependability. Security is assessed by theprobability of an unwanted command occurring, anddependability is assessed by the probability of missing acommand. The required degree of security anddependability is related to the mode of operation, thecharacteristics of the communication medium and theoperating standards of the particular power authority.

Typical design objectives for teleprotection systems arenot more than one incorrect trip per 500 equipmentyears and less than one failure to trip in every 1000attempts, or a delay of more than 50msec should notoccur more than once per 10 equipment years. Toachieve these objectives, special emphasis may beattached to the security and dependability of theteleprotection command for each mode of operation inthe system, as follows.

8.5.1 Performance Requirements – Intertripping

Since any unwanted command causes incorrect tripping,very high security is required at all noise levels up to themaximum that might ever be encountered.

8.5.2 Performance Requirements – Permissive Tripping

Security somewhat lower than that required forintertripping is usually satisfactory, since incorrecttripping can occur only if an unwanted commandhappens to coincide with operation of the protectionrelay for an out-of-zone fault.

For permissive over-reach schemes, resetting after acommand should be highly dependable to avoid anychance of maloperations during current reversals.

8.5.3 Performance Requirements – Blocking Schemes

Low security is usually adequate since an unwantedcommand can never cause an incorrect trip. Highdependability is required since absence of the commandcould cause incorrect tripping if the protection relayoperates for an out-of-zone fault.

Typical performance requirements are shown in Figure 8.2.

8.6 TRANSMISSION MEDIAINTERFERENCE AND NOISE

The transmission media that provide the communicationlinks involved in protection signalling are:

a. private pilotsb. rented pilots or channelsc. power line carrierd. radioe. optical fibres

Historically, pilot wires and channels (discontinuous pilotwires with isolation transformers or repeaters along theroute between signalling points) have been the mostwidely used due to their availability, followed by PowerLine Carrier Communications (PLCC) techniques andradio. In recent years, fibre-optic systems have becomethe usual choice for new installations, primarily due totheir complete immunity from electrical interference.The use of fibre-optic cables also greatly increases thenumber of communication channels available for each

physical fibre connection and thus enables morecomprehensive monitoring of the power system to beachieved by the provision of a large number ofcommunication channels.

8.6.1 Private Pilot Wires and Channels

Pilot wires are continuous copper connections betweensignalling stations, while pilot channels arediscontinuous pilot wires with isolation transformers orrepeaters along the route between signalling stations.They may be laid in a trench with high voltage cables,laid by a separate route or strung as an open wire on aseparate wood pole route.

Distances over which signalling is required varyconsiderably. At one end of the scale, the distance may beonly a few tens of metres, where the devices concerned arelocated in the same substation. For applications on EHVlines, the distance between devices may be between 10-100km or more. For short distances, no special measuresare required against interference, but over longer distances,special send and receive relays may be required to boostsignal levels and provide immunity against inducedvoltages from power circuits, lightning strikes to groundadjacent to the route, etc. Isolation transformers may alsohave to be provided to guard against rises in substationground potential due to earth faults.

The capacity of a link can be increased if frequencydivision multiplexing techniques are used to run parallelsignalling systems, but some Utilities prefer the link to beused only for protection signalling.

Private pilot wires or channels can be attractive to anUtility running a very dense power system with shortdistances between stations.

8.6.2 Rented Pilot Wires and Channels

These are rented from national communicationauthorities and, apart from the connection from therelaying point to the nearest telephone exchange, therouting will be through cables forming part of thenational communication network.

An economic decision has to be made between the useof private or rented pilots. If private pilots are used, theowner has complete control, but bears the cost ofinstallation and maintenance. If rented pilots are used,most of these costs are eliminated, but fees must be paidto the owner of the pilots and the signal path may bechanged without warning. This may be a problem inprotection applications where signal transmission timesare critical.

The chance of voltages being induced in rented pilots issmaller than for private pilots, as the pilot route isnormally not related to the route of the power line withwhich it is associated. However, some degree of security

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Figure 8.2: Typical performance requirements for protection signalling when the communication link is subjected to noise

0

0.020.01 10

10

10-5

10-4

10-3

10-2

0.03

0.06

0.040.05

Sec

Intertrip

Blocking

CC

Analogue Digital

DigitalAnalogue

TOPOP

IntertripTntertripT - 0.04secPUC -1.00E-03P -1.00E-01

TOPTOPT - 0.015secP

OP-1.00E-01

PMCP -1.00E-01

TT - 0.015sec-2.00E-02-1.00E-01

IntertripTntertrip

- 0.04secPP

TTPP -1.00E-01

T - 0.015sec

P

TT - Maximum operating timeimum operating timeimum operating timeimum operating timeimum operating timeimum operating time

ª - UC )%PMCP

Dependability ª 100(1-PMCP )%

TOPOP

Figure 8.2: Typical performance requirementsfor protection signalling when the

communication link is subjected to noise

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and protection against induced voltages must be builtinto signalling systems. Electrical interference fromother signalling systems, particularly 17, 25 and 50Hzringing tones up to 150V peak, and from noise generatedwithin the equipment used in the communicationnetwork, is a common hazard. Similarly, the signallingsystem must also be proof against intermittent short andopen circuits on the pilot link, incorrect connection of 50volts d.c. across the pilot link and other similar faults.

Station earth potential rise is a significant factor to betaken into account and isolation must be provided toprotect both the personnel and equipment of thecommunication authority.

The most significant hazard to be withstood by aprotection signalling system using this medium ariseswhen a linesman inadvertently connects a lowimpedance test oscillator across the pilot link that cangenerate signalling tones. Transmissions by such anoscillator may simulate the operating code or tonesequence that, in the case of direct intertrippingschemes, would result in incorrect operation of thecircuit breaker.

Communication between relaying points may be over atwo-wire or four-wire link. Consequently the effect of aparticular human action, for example an incorrectdisconnection, may disrupt communication in onedirection or both.

The signals transmitted must be limited in both level andbandwidth to avoid interference with other signallingsystems. The owner of the pilots will impose standards

in this respect that may limit transmission capacityand/or transmission distance.

With a power system operating at, say, 132kV, whererelatively long protection signalling times are acceptable,signalling has been achieved above speech together withmetering and control signalling on an established controlnetwork. Consequently the protection signalling wasachieved at very low cost. High voltage systems (220kVand above) have demanded shorter operating times andimproved security, which has led to the renting of pilotlinks exclusively for protection signalling purposes.

8.6.3 Power Line Carrier Communications Techniques

Where long line sections are involved, or if the routeinvolves installation difficulties, the expense of providingphysical pilot connections or operational restrictionsassociated with the route length require that othermeans of providing signalling facilities are required.

Power Line Carrier Communications (PLCC) is a techniquethat involves high frequency signal transmission alongthe overhead power line. It is robust and thereforereliable, constituting a low loss transmission path that isfully controlled by the Utility.

High voltage capacitors are used, along with drainagecoils, for the purpose of injecting the signal to andextracting it from the line. Injection can be carried outby impressing the carrier signal voltage between oneconductor and earth or between any two phaseconductors. The basic units can be built up into a highpass or band pass filter as shown in Figure 8.3.

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Figure 8.3: Typical phase-to-phase coupling equipmentFigure 8.3: Typical phase-to-phase coupling equipment

Seriestuningunit

To E/M VTTo E/M VT

To line To station

Line trap

Capacitor VT

Shunt filter unit

75 ohms Coaxial cableTo HF equipment

The high voltage capacitor is tuned by a tuning coil topresent a low impedance at the signal frequency; theparallel circuit presents a high impedance at the signalfrequency while providing a path for the powerfrequency currents passed by the capacitor.

The complete arrangement is designed as a balanced orunbalanced half-section band pass filter, according towhether the transmission is phase-phase or phase-earth;the power line characteristic impedance, between 400 and600 ohms, determines the design impedance of the filter.

It is necessary to minimize the loss of signal into other partsof the power system, to allow the same frequency to beused on another line. This is done with a 'line trap' or 'wavetrap', which in its simplest form is a parallel circuit tuned topresent a very high impedance to the signal frequency. It isconnected in the phase conductor on the station side of theinjection equipment. The complete carrier couplingequipment is shown in Figure 8.4.

The single frequency line trap may be treated as anintegral part of the complete injection equipment toaccommodate two or more carrier systems. However,difficulties may arise in an overall design, as, at certainfrequencies, the actual station reactance, which isnormally capacitive, will tune with the trap, which isinductive below its resonant frequency; the result will bea low impedance across the transmission path,preventing operation at these frequencies. This situationcan be avoided by the use of an independent 'doublefrequency' or 'broad-band' trap.

The coupling filter and the carrier equipment areconnected by high frequency cable of preferredcharacteristic impedance 75 ohms. A matchingtransformer is incorporated in the line coupling filter tomatch it to the hf cable. Surge diverters are fitted toprotect the components against transient over voltages.

The attenuation of a channel is of prime importance inthe application of carrier signalling, because itdetermines the amount of transmitted energy availableat the receiving end to overcome noise and interferingvoltages. The loss of each line terminal will be 1 to 2dBthrough the coupling filter, a maximum of 3dB throughits broad-band trap and not more than 0.5dB per 100metres through the high frequency cable.

An installation of PLCC equipment including capacitorvoltage transformers and line traps, in a line-to-lineinjection arrangement, is shown in Figure 8.4.

The high frequency transmission characteristics of powercircuits are good the loss amounting to 0.02 to 0.2dB perkilometre depending upon line voltage and frequency.Line attenuation is not affected appreciably by rain, butserious increase in loss may occur when the phaseconductors are thickly coated with hoar-frost or ice.Attenuations of up to three times the fair weather valuehave been experienced. Receiving equipment commonlyincorporates automatic gain control (AGC) tocompensate for variations in attenuation of signals.

High noise levels arise from lightning strikes and systemfault inception or clearance. Although these are of shortduration, lasting only a few milliseconds at the most, theymay cause overloading of power line carrier receivingequipment. Signalling systems used for intertripping inparticular must incorporate appropriate security features toavoid maloperation. The most severe noise levels areencountered with operation of the line isolators, and thesemay last for some seconds. Although maloperation of theassociated teleprotection scheme may have littleoperational significance, since the circuit breaker at one endat least is normally already open, high security is generallyrequired to cater for noise coupled between parallel lines orpassed through line traps from adjacent lines.

Signalling for permissive intertrip applications needsspecial consideration, as this involves signalling througha power system fault. The increase in channelattenuation due to the fault varies according to the typeof fault, but most power authorities select a nominalvalue, usually between 20 and 30dB, as an applicationguide. A protection signal boost facility can be employedto cater for an increase in attenuation of this order ofmagnitude, to maintain an acceptable signal-to-noiseratio at the receiving end, so that the performance of theservice is not impaired.

Most direct intertrip applications require signalling overa healthy power system, so boosting is not normallyneeded. In fact, if a voice frequency intertrip system isoperating over a carrier bearer channel, the dynamicoperating range of the receiver must be increased toaccommodate a boosted signal. This makes it lessinherently secure in the presence of noise during aquiescent signalling condition.

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Figure 8.4: Carrier coupling equipment

8.6.4 Radio Channels

At first consideration, the wide bandwidth associatedwith radio frequency transmissions could allow the useof modems operating at very high data rates. Protectionsignalling commands could be sent by serial codedmessages of sufficient length and complexity to givehigh security, but still achieve fast operating times. Inpractice, it is seldom economic to provide radioequipment exclusively for protection signalling, sostandard general-purpose telecommunications channelequipment is normally adopted.

Typical radio bearer equipment operates at themicrowave frequencies of 0.2 to 10GHz. Because of therelatively short range and directional nature of thetransmitter and receiver aerial systems at thesefrequencies, large bandwidths can be allocated withoutmuch chance of mutual interference with other systems.

Multiplexing techniques allow a number of channels toshare the common bearer medium and exploit the largebandwidth. In addition to voice frequency channels, widerbandwidth channels or data channels may be available,dependent on the particular system. For instance, inanalogue systems using frequency division multiplexing,normally up to 12 voice frequency channels are groupedtogether in basebands at 12-60kHz or 60-108kHz, butalternatively the baseband may be used as a 48kHz signalchannel. Modern digital systems employing pulse codemodulation and time division multiplexing usually providethe voice frequency channels by sampling at 8kHz andquantising to 8 bits; alternatively, access may be availablefor data at 64kbits/s (equivalent to one voice frequencychannel) or higher data rates.

Radio systems are well suited to the bulk transmission ofinformation between control centres and are widely usedfor this. When the route of the trunk data networkcoincides with that of transmission lines, channels canoften be allocated for protection signalling. Moregenerally, radio communication is between majorstations rather than the ends of individual lines, becauseof the need for line-of-sight operation between aerialsand other requirements of the network. Roundaboutroutes involving repeater stations and the addition ofpilot channels to interconnect the radio installation andthe relay station may be possible, but overalldependability will normally be much lower than for PLCCsystems in which the communication is direct from oneend of the line to the other.

Radio channels are not affected by increased attenuationdue to power system faults, but fading has to be takeninto account when the signal-to-noise ratio of aparticular installation is being considered.

Most of the noise in such a protection signalling systemwill be generated within the radio equipment itself.

A polluted atmosphere can cause radio beam refraction thatwill interfere with efficient signalling. The height of aerialtower should be limited, so that winds and temperaturechanges have the minimum effect on their position.

8.6.5 Optical Fibre Channels

Optical fibres are fine strands of glass, which behave aswave guides for light. This ability to transmit light overconsiderable distances can be used to provide opticalcommunication links with enormous informationcarrying capacity and an inherent immunity toelectromagnetic interference.

A practical optical cable consists of a central opticalfibre which comprises core, cladding and protectivebuffer coating surrounded by a protective plasticoversheath containing strength members which, in somecases, are enclosed by a layer of armouring.

To communicate information a beam of light ismodulated in accordance with the signal to betransmitted. This modulated beam travels along theoptical fibre and is subsequently decoded at the remoteterminal into the received signal. On/off modulation ofthe light source is normally preferred to linearmodulation since the distortion caused by non-linearitiesin the light source and detectors, as well as variations inreceived light power, are largely avoided.

The light transmitter and receiver are usually laser or LEDdevices capable of emitting and detecting narrow beamsof light at selected frequencies in the low attenuation850, 1300 and 1550 nanometre spectral windows. Thedistance over which effective communications can beestablished depends on the attenuation and dispersion ofthe communication link and this depends on the typeand quality of the fibre and the wavelength of theoptical source. Within the fibre there are many modes ofpropagation with different optical paths that causedispersion of the light signal and result in pulsebroadening. The degrading of the signal in this way canbe reduced by the use of 'graded index' fibres that causethe various modes to follow nearly equal paths. Thedistance over which signals can be transmitted issignificantly increased by the use of 'monomode' fibresthat support only one mode of propagation.

With optical fibre channels, communication at data ratesof hundreds of megahertz is achievable over a few tens ofkilometres, whilst greater distances require the use ofrepeaters. An optical fibre can be used as a dedicated linkbetween two terminal equipments, or as a multiplexedlink that carries all communication traffic such as voice,telecontrol and protection signalling. In the latter casethe available bandwidth of a link is divided by means oftime division multiplexing (T.D.M.) techniques into anumber of channels, each of 64kbits/s (equivalent to one

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voice frequency channel which typically uses an 8-bitanalogue-to-digital conversion at a sampling rate of8kHz). A number of Utilities sell surplus capacity on theirlinks to telecommunications operators. The trend ofusing rented pilot circuits is therefore being reversed,with the Utilities moving back towards ownership of thecommunication circuits that carry protection signalling.

The equipments that carry out this multiplexing at eachend of a line are known as 'Pulse Code Modulation'(P.C.M.) terminal equipments. This approach is the oneadopted by telecommunications authorities and someUtilities favour its adoption on their private systems, foreconomic considerations.

Optical fibre communications are well established in theelectrical supply industry. They are the preferred meansfor the communications link between a substation and atelephone exchange when rented circuits are used, astrials have shown that this link is particularly susceptibleto interference from power system faults if copperconductors are used. Whilst such fibres can be laid incable trenches, there is a strong trend to associate themwith the conductors themselves by producing compositecables comprising optical fibres embedded within theconductors, either earth or phase. For overhead lines useof OPGW (Optical Ground Wire) earth conductors is verycommon, while an alternative is to wrap the opticalcable helically around a phase or earth conductor. Thislatter technique can be used without restringing of theline.

8.7 S IGNALLING METHODS

Various methods are used in protection signalling; not allneed be suited to every transmission medium. Themethods to be considered briefly are:

a. D.C. voltage step or d.c. voltage reversals

b. plain tone keyed signals at high and voicefrequencies

c. frequency shift keyed signals involving two or moretones at high and voice frequencies

General purpose telecommunications equipmentoperating over power line carrier, radio or optical fibremedia incorporate frequency translating or multiplexingtechniques to provide the user with standardisedcommunication channels. They have a nominalbandwidth/channel of 4kHz and are often referred to asvoice frequency (vf) channels. Protection signallingequipments operating at voice frequencies exploit thestandardisation of the communication interface. Wherevoice frequency channels are not available or suitable,protection signalling may make use of a medium orspecialised equipment dedicated entirely to thesignalling requirements.

Figure 8.5 illustrates the communication arrangementscommonly encountered in protection signalling.

8.7.1 D.C. Voltage Signalling

A d.c. voltage step or d.c. voltage reversals may be usedto convey a signalling instruction between protectionrelaying points in a power system, but these are suitedonly to private pilot wires, where low speed signalling isacceptable, with its inherent security.

8.7.2 Plain Tone Signals

Plain high frequency signals can be used successfully forthe signalling of blocking information over a power line.A normally quiescent power line carrier equipment canbe dedicated entirely to the transfer to teleprotectionblocking commands. Phase comparison power linecarrier unit protection schemes often use suchequipment and take advantage of the very high speedand dependability of the signalling system. The specialcharacteristics of dedicated 'on/off' keyed carrier systemsare discussed later. A relatively insensitive receiver isused to discriminate against noise on an amplitude basis,and for some applications the security may besatisfactory for permissive tripping, particularly if thenormal high-speed operation of about 6ms is sacrificedby the addition of delays. The need for regular reflextesting of a normally quiescent channel usually precludesany use for intertripping.

Plain tone power line carrier signalling systems areparticularly suited to providing the blocking commandsoften associated with the protection of multi-endedfeeders, as described in Chapter 13. A blockingcommand sent from one end can be receivedsimultaneously at all the other ends using a single powerline carrier channel. Other signalling systems usuallyrequire discrete communication channels between eachof the ends or involve repeaters, leading to decreaseddependability of the blocking command.

Plain voice frequency signals can be used for blocking,permissive intertrip and direct intertrip applications forall transmission media but operation is at such a lowsignal level that security from maloperation is not verygood. Operation in the 'tone on' to 'tone off' mode givesthe best channel monitoring, but offers little security; toobtain a satisfactory performance the output must bedelayed. This results in relatively slow operation: 70milliseconds for permissive intertripping, and 180milliseconds for direct intertripping.

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8.7.3 Frequency Shift Keyed Signals

Frequency shift keyed high frequency signals can beused over a power line carrier link to give shortoperating times (15 milliseconds for blocking andpermissive intertripping, 20 milliseconds for directintertripping) for all applications of protectionsignalling. The required amount of security can beachieved by using a broadband noise detector tomonitor the actual operational signalling equipment.

Frequency shift keyed voice frequency signals can beused for all protection signalling applications over alltransmission media. Frequency modulation techniquesmake possible an improvement in performance, becauseamplitude limiting rejects the amplitude modulationcomponent of noise, leaving only the phase modulationcomponents to be detected.

The operational protection signal may consist of tonesequence codes with, say, three tones, or a multi-bitcode using two discrete tones for successive bits, or of asingle frequency shift.

Modern high-speed systems use multi-bit code or singlefrequency shift techniques. Complex codes are used to

give the required degree of security in direct intertripschemes: the short operating times needed may result inuneconomical use of the available voice frequencyspectrum, particularly if the channel is not exclusivelyemployed for protection signalling. As noise power isdirectly proportional to bandwidth, a large bandwidthcauses an increase in the noise level admitted to thedetector, making operation in the presence of noise moredifficult. So, again, it is difficult to obtain both highdependability and high security.

The signal frequency shift technique has advantageswhere fast signalling is needed for blocked distance andpermissive intertrip applications. It has little inherentsecurity, but additional circuits responsive to every typeof interference can give acceptable security. This systemdoes not require a channel capable of high transmissionrates, as the frequency changes once only; thebandwidth can therefore be narrower than in codedsystems, giving better noise rejection as well as beingadvantageous if the channel is shared with telemetryand control signalling, which will inevitably be the caseif a power line carrier bearer is employed.

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Figure 8.5: Communication arrangements commonly encountered in protection signalling

Voicefrequency

Carrierfrequency

shift

On/offkeyedcarrier

Digital

Optical

Protectionrelay

scheme

Protectionsignallingequipment

PCMprimary

multiplex

Frequencydivision

multiplex

Radiotransmitter

Opticaltransmitter

Communicationequipment

Power linecarrier

communicationchannel Power line carrier Power line carrier

Pilot wires

Pilot channel

Transmission media

Optical fibregeneral purpose

Optical fibrededicated

Radio

Figure 8.5: Communication arrangements commonly encountered in protection signalling